Generally, an electrographic image forming apparatus, such as a laser printer and a digital copying machine, uses an exposure unit that turns on and off a laser beam correspondingly to data of an image to be printed and scans a photosensitive member with this laser beam, thereby forming an electrostatic latent image on the photosensitive member.
The exposure unit includes a light source such as a laser diode, and a coupling lens (collimate lens). The coupling lens is disposed at the front of the laser diode for converting a laser light into a laser beam (in this specification, with respect to the light source device, an advancing direction in an optical-axis direction, i.e., downstream, is referred to as a “front”).
In order to form an excellent image, the laser light needs to be converged into a small point on the exposure surface of the photosensitive member. Therefore, it is necessary to strictly determine a distance in an optical-axis direction between the laser diode and the coupling lens on the order of several μm to several hundreds of μm.
Thus, in order to strictly position the light source and the coupling lens, the position of the light source and the coupling lens is adjusted in two stages, i.e., a rough adjustment and a fine adjustment. For example, in an exposure unit disclosed in Patent Document 1, optical axes of a laser diode and a collimate lens are adjusted (i.e., the position in an X-Y direction orthogonal to the optical axis is aligned) with respect to each other, and in this state, both components, i.e. the laser diode and the collimate lens, are fixed to a holding member made of an aluminum plate. Thereafter, a portion of the aluminum plate holding the laser diode is deflected by feeding of a screw (see numeral 370 in FIG. 5) so as to finely adjust the position of the laser diode in the optical-axis direction (referred as a Z-direction).
Patent Document 1: Japanese Published Unexamined Patent Application No. 2004-163463 (FIGS. 5 and 6, U.S. Pat. No. 7,349,166)
In the method described in Patent Document 1, when the position of the laser diode is finely adjusted in the Z-direction by the feeding of the screw, the rotation force by which the screw is rotated may act on the aluminum plate holding the laser diode to misalign the optical axes of the light source and the coupling lens. Such misalignment of the optical axes enlarges aberration of a beam shaped by the coupling lens. This may result in a possibility that a laser beam cannot be converged into a desired small point.